1. Field of the Invention
This invention relates generally to active high pass filters, and more particularly to an asymmetric digital subscriber line (ADSL) receiver high pass filter (OPF) architecture.
2. Description of the Prior Art
An asymmetric digital subscriber line (ADSL) receiver comprises a large number of operational amplifiers (Op Amps) and is generally implemented in an application specific integrated circuit (ASIC) to conserve on space, provide for reduced power consumption, and to minimize manufacturing costs. The number of Op Amps used to implement the ASIC is important because the ADSL receiver high pass filter (HPF) must be a low noise circuit, implying a high supply current, that increases power dissipation and consequently, silicon area requirements.
Programmable gain amplification (PGA) is generally used to provide acceptable noise performance and also to avoid saturation of Op Amps that are employed in ADSL receivers. The gain is high for low receiving levels and low for high receiving levels.
Known ADSL receivers employ a large number of Op Amps due to the architecture of the associated BPF 100 shown in FIG. 1 that uses PGA. This well-known HPF architecture is based on two third-order Elliptic filters. The first third-order Elliptic filter includes first-order HPF stage 102 and second-order HPF notch stage 104. The second third-order Elliptic filter includes first-order HPF stage 106 and second-order HPT notch stage 108. The second third-order Elliptic filter 106, 108 is a copy of the first third-order Elliptic filter 102, 104.
Each of the third-order Elliptic filters shown in FIG. 1 is implemented using the HPF circuit 200 shown in FIG. 2. Op Amp 202 is used to implement a first-order BPF transfer function. Op Amps 204 and 206 are used to implement a second order HPF transfer function having a notch (imaginary zero). HPF circuit 200 is identical for each third-order HPF, as stated herein before.
The first third-order Elliptic HPF 102, 104 and the second third-order Elliptic HPF 106, 108 provide for rejection of the transmitter signal in the bandwidth between 30 kHz–180 kHz, and provides gain for frequencies above 180 kHz. The value of gain is dependent on the received signal level.
The PGA associated with the first first-order HPF stage 102 and the first second-order stage BPF 104 effectively provide the level of −150 dBm/Hz noise. This low level of noise is necessary to avoid limitations caused by amplifier front-end (AFE) noise. The PGA in each stage of HPF 100 is also required to minimize the noise caused by the respective HPF resistors and Op Amps. The second third-order filter 106, 108 provides additional rejection of the transmitted signal in the bandwidth 30 kHz–180 kHz.
The well known HPF 100 is problematic however, in that despite the fact that each of the third-order filters 102/104 and 106/108 are Elliptic, the total sixth-order HPF 100 is not an Elliptic filter, and undesirably produces high Q factors for each of the second-order stages 104 and 108. This solution can also be seen to require two Op Amps and the addition of three capacitors for each second-order stage 104 and 108.
It is therefore both advantageous and desirable in view of the foregoing, to provide an ADSL receiver HPF architecture that reduces the number of Op Amps employed in the ADSL receiver HPF.